The present invention relates to novel tetrahydrofluorenyl containing Group 4 metallocenes as catalysts for the polymerization of olefins. It relates more specifically to novel silylene or ethylene bis(tetrahydrofluorenyl) metallocenes and to their use for polymerizing olefins. Finally, it relates to a process for preparing said metallocenes.
The use of bis(fluorenyl) metallocenes for polymerizing olefins has already been described. However most of these metallocenes are unstable due to internal ligand rearrangements and cannot be used in an economic way.
It is an object of the present invention to solve the above problem by providing novel bis(tetrahydrofluorenyl) metallocenes useful as catalysts for olefin polymerization, which are stable. More particularly it is an object of the invention to provide bis(tetrahydrofluorenyl) metallocenes which are able to produce, with a particularly high activity, polyethylene having high molecular weight or polyethylene having a broad or even a bimodal molecular weight distribution. It is another object of the present invention to provide a process for polymerization of olefins by means of said metallocenes and a process for preparing said novel metallocenes.
The invention is thus related to novel metallocenes represented by the general formula 
wherein:
M represents a group 4 transition metal,
X and Xxe2x80x2 represent a halogen atom,
R represents an alkyl or aryl group containing from 1 to 10 carbon atoms or a hydrogen atom, and
Y represents an ethylene group or a silylene group of formula SiR1R2 where R1 and R2 represent each independently an alkyl or aryl radical containing from 1 to 10 carbon atoms,
said metallocene being usable for the polymerization of olefins.
The invention also relates to a process for the preparation of metallocenes according to the invention comprising the following steps:
a) preparation of 2-benzylcyclohexanone or substituted derivatives thereof by reaction of 2-sodio-cyclohexanone with benzyl chloride or substituted derivatives thereof,
b) production of the tetrahydrofluorenyle compounds by reaction of the 2-benzylcyclohexanones with aluminum trichloride,
c) production of the ethylene or dialkylsilylene bridged ligands by reacting the tetrahydrofluorenyle compounds successively with butyllithium and 1,2-dibromoethane or dichlorodialkyl- or arylsilane, and
d) production of the bridged bis(tetrahydrofluorenyl) metallocene by reacting the said ligands successively with butyllithium and metal tetrahalide.
According to a first aspect, the present invention relates to novel metallocenes of the above general formula.
Preferably the transition metal M is selected from hafnium and zirconium. Most preferably the transition metal is zirconium.
The halogen atoms X and Xxe2x80x2 are preferably chlorine or bromine atoms and most preferably they are both chlorine atoms.
The radical R represents preferably an alkyl group containing from 1 to 3 carbon atoms or a hydrogen atom.
The silylene group is advantageously such that the radicals R1 and R2 are alkyl radicals and more particularly alkyl radicals containing from 1 to 3 carbon atoms.
The novel metallocenes according to the present invention are very stable. They are useful as catalysts for the polymerization of olefins. The reaction is carried out by contacting said olefins with the said metallocene under polymerization conditions. It can be carried out in solution or in suspension in a hydrocarbon diluant or in suspension in the, or one of the, monomer(s) maintained in the liquid form or in the gas phase. The polymerization conditions are well known by the man of ordinary skill in the art.
The metallocenes according to the invention can be used in combination with one another. They can also be used in combination with aluminoxanes. Methylaluminoxane is preferred. They can also be used in combination with an ionizing agent. This ionizing agent can be chosen from the compounds comprising a first part which has the properties of a Lewis acid and which is capable of ionizing the metallocene and a second part that is inert towards the ionized metallocene. Examples of ionizing agents are triphenylcarbenium tetrakis(pentafluorophenyl) borate, N,Nxe2x80x2-dimethyl anilinium tetrakis(pentafluorophen l) borate, tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, tri(pentafluorophenyl)boron, triphenylboron, trimethylboron, tri(trimethylsilyl)borate and organoboroxines.
During the polymerization, organometallic compounds may be added to the polymerization medium as cocatalysts and/or poison scavengers. They can be selected from organometallic compounds of lithium, magnesium, zinc, aluminum or tin. The best results are obtained with organoaluminium compounds and in particular with trialkylaluminium compounds.
The olefins to be polymerized can be chosen from those containing up to 20, preferably up to 8 carbon atoms per molecule. The olefin is preferably ethylene. The metallocenes according to the present invention may be used for the homopolymerization of one of these olefins or for the copolymerizationxe2x80x94random or block copolymerizationxe2x80x94of one of these olefins with one or more comonomers. The preferred comonomers of ethylene are propylene, butene, hexene and their mixtures.
It has been noted that metallocenes according to the invention wherein both R are alkyl makes it possible to obtain polyethylene polymers with very high activity. It is also noted that metallocenes according to the invention in which Y is dialkyl or diaryl silylene are able to produce polyethylenes of high molecular weight. The use of metallocenes wherein both R are hydrogen atoms makes it possible to obtain of polyethylenes having a broad or a bimodal distribution of the molecular weight. Such polymers are particularly easily molded according to any known process.
Metallocenes of formula 
wherein R is chosen from alkyl having from 1 to 3 carbon atoms, M is zirconium, X and Xxe2x80x2 are chlorine atoms and R1 and R2 are each alkyl radical containing 1 to 3 carbon atoms are particularly advantageous for the production, with high activity, of polyethylene having high molecular weight.
According to a second aspect, the present invention relates to a process for producing the novel metallocenes according to the present invention.
The said process comprises the following steps
a) preparation of 2-benzylcyclohexanone or the substituted derivatives thereof by reaction of 2-sodio-cyclohexanone with benzyl chloride or substituted derivatives thereof,
b) production of the tetrahydrofluorenyle compounds by reaction of the 2-benzylcyclohexanones with aluminum trichloride,
c) production of the ethylene or dialkylsilylene bridged ligands by reacting the tetrahydrofluorenyle compounds successively with butyllithium and 1,2-dibromoethane or dichlorodialkyl or arylsilane, and
d) production of the bridged bis(tetrahydrofluorenyl) metallocenes by reacting the said ligands successively with butyllithium and metal tetrahalide.
The general conditions of steps (a) and (b) are disclosed in Bull. Chem. Soc. Fr. 1952, 786 and 1953, 75 which are incorporated herein by reference.
In step (a), 2-sodio-cyclohexanone and the benzyl chlorides are reacted in about equivalent molar quantities.
Preferably, step (b) is carried out by adding about 3 moles of aluminium trichloride per mole of benzylcyclohexanone.
Step (c) is preferably carried by adding about one equivalent of butyllithium and a half equivalent of 1,2 dibromoethane or dichlorodialkyl or arylsilane per equivalent of tetrahydrofluorenyle compound.
Step (d) is most often carried out by reacting first about two equivalents of butyllithium per equivalent of the bis(tetrahydrofluorenyl) ligand to obtain a solid that is then treated with one equivalent metal tetrahalide per equivalent of the tetrahydrofluorenyle ligand.
Preferably, reaction step (a) is carried out in an inert solvent, for example an ether and more particularly anhydrous diethyl ether. Reaction step (a) is usually carried out at a temperature in the range of 25-85xc2x0 C. At the end of reaction of step (a), the solvent is generally removed in order to separate the produced benzylcyclohexanone.
Preferably, the step (b) is carried out in an inert solvent such as hexane or other alkane, most often at about 50-90xc2x0 C. At the end of the reaction, the tetrahydrofluorenyle compound is usually isolated in the form of white crystals by crystallization from methanol.
Preferably, step (c) is carried out in an inert solvent, such as an ether, most often in tetrahydrofuran. The temperature is usually in the range of about minus 10xc2x0 C. to about room temperature. Temperature of about 0xc2x0 C. is the most advantageous. The reaction is usually carried out by adding first the butyllithium to the tetrahydrofluorenyle compound. The reaction medium was then usually stirred for a period of from 2 to 10 hours before adding the 1,2 dibromoethane or the dichlorodialkyl or arylsilane. The resulting bis(tetrahydrofluorenyl) ligand is then advantageously separated from its preparation medium and recrystallized.
Step (d) is preferably carried out in an inert solvent, such as diethyl ether at a temperature of between minus 10xc2x0 C. to 10xc2x0 C., preferably at about 0xc2x0 C. by addition of about two equivalents of butyllithium per equivalent of the bis(tetrahydrofluorenyl) ligand. The obtained solid was then usually reacted, after isolation, with about one equivalent of the metal tetrahalide. After removal of the solvent, the solid metallocene is isolated preferably by extraction into dichloromethane, concentration and cooling to xe2x88x9220xc2x0 C.